4.1.1 Scaled scores
Scaled scores (with a mean of 10 and a standard deviation of 3) are reported here because they are part of the standard scoring of CELF-4, and thus, clinicians will be familiar with this scoring method. They are presented in part as a comparison to the other scoring methods employed in this study, which may not be as familiar to practitioners. The distributions of scaled scores within the two groups are illustrated in figures 4.1 and 4.2 below. There are significant differences between groups in the distributions of scaled scores on the CELF-4 Sentence Recall subtest. The scaled score represents the child’s score for all sentences, not just the subset of 9 sentences that this study is based on.
Figure 4.1 Distribution of scaled scores within DLD group for CELF-4 Recalling Sentences subtest (all 18 sentences)
37 Figure 4.2 Distribution of scaled scores within TD control group for CELF-4 Recalling Sentences subtest (all 18 sentences)
All 21 children in the TD control group received a scaled score denoting average, high average or above average (group Min=8, Max=15) while the majority of children in the DLD group (15 out of 21) received a scaled score at least 1 SD below the standardized mean, signifying a lower than average score; six children with DLD got a scaled score of average, and none scored at or higher than 1 SD above the mean (group Min=1, Max=12). The average scaled score for the DLD group (M=5.48, SD=3.25) represents a below average score, while the average scaled score for the control group (M=11.10, SD=2.07) represents an average score.
The mean difference between the two groups is significant (t(34)=-6.68, p<0.001) with a large effect size (d=2.29). Levene’s test indicated unequal variances (F=5.51, p=0.024), so degrees of freedom were adjusted from 40 to 34.
4.1.2 Exact repetitions
An independent-samples t-test was conducted using scoring method 1 for exact repetitions, with a possible score range of 0 to 9. This test indicated that children with DLD gave on average significantly fewer exact repetitions of the nine sentences (M=2.48, SD=1.78) than
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the TD control group (M=5.81, SD=1.47), (t(40)=-6.62, p<0.001). As was the case for the comparison of scaled scores, this comparison yielded a large effect size (d=2.09).
A visual investigation of differences between groups for each sentence was facilitated by calculating the percentage of children in each group who gave an exact repetition of each sentence. This is depicted in table 4.3.
Table 4.3 Percentage exact repetitions by group (N=21) and sentence (5-13)
Sentence Percentage exact repetitions
PT level Syll. length DLD group Control group
(5) 3 13 71% 100%
(6) 3 14 24% 86%
(7) 2 15 48% 90%
(8) 3 17 48% 81%
(9) 3 16 24% 81%
(10) 3 21 14% 52%
(11) 4 22 10% 33%
(12) 5 22 10% 24%
(13) 5 24 0% 33%
The control group performed better than the DLD group on all sentences. More than half of the TD children managed to give an exact repetition up to and including sentence (10). In contrast, the only sentence that a majority of the children with DLD were able to repeat with no errors was (5) (the starting point for children aged 9-12 years). There is a general trend in both groups of the proportion of exact repetitions decreasing from sentence (5) to (13), as sentences become longer and use more complex syntax. One notable exception to this trend was (6) for the DLD group, which yielded a very low proportion of exact repetitions
compared to sentences of approximately the same length and complexity.
The three most difficult sentences for both groups according to this metric were (11), (12) and (13), which are the longest sentence (along with sentence [10]) and the most syntactically complex sentences according to the PT hierarchy.
4.1.3 Total number of errors
An independent-samples t test was conducted using scoring method 2 for total error count (with a possible score range of 0 to 97) to further investigate differences between the two
39 groups. The DLD group made on average significantly more errors (M=30.19, SD=14.18), than the TD control group (M=7.43, SD=5.07; t(25)=6.93; p<0.001). Levene’s test indicated unequal variances (F=22.36, p<0.001), so degrees of freedom were adjusted from 40 to 25.
There was a much larger variation of scores within the DLD group than within the control group, with the standard deviation of the DLD group’s distribution of scores being nearly three times as large as for the control group. Similar to the other two analyses for mean group differences, a large effect size was found (d=2.77).
4.1.4 Error rate
Lastly, an independent-samples t test was conducted using scoring method 3 for error rate to further confirm differences between the groups. Not surprisingly, this also yielded a
significant between-groups difference, with the DLD group having an overall error rate of 29% (M=0.29, SD=0.15) and the TD control group having an overall error rate of 7%
(M=0.07, SD=0.05). Levene’s test indicated unequal variances (F=27.25, p<0.001), so degrees of freedom were adjusted from 40 to 24. Once again there was a larger variation in performance within the DLD group than within the control group. The level of significance and effect size was large, similar to the other scoring methods (t(24)=6.54, p<0.001, d=2.67).
Since the nine sentences included in the analysis are of varying length, we cannot easily compare them by examining number of errors per sentence. It is not possible to make as many errors in a shorter sentence as in a longer sentence, errors being mainly calculated at the word-level. Error rate was defined as the total number of errors divided by number of words per sentence, a suitable metric for examining the relative difficulty of the sentences. The average error rate within groups for each sentence is visually depicted in table 4.4 below.
Table 4.4 Error rate per sentence by group
Group Sentence
5 6 7 8 9 10 11 12 13
DLD Mean 0.08 0.24 0.19 0.09 0.29 0.29 0.46 0.44 0.50
SD 0.18 0.23 0.23 0.11 0.28 0.27 0.33 0.26 0.23
Min 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.12
Max 0.75 1.00 0.88 0.36 0.90 1.00 1.00 1.00 1.00
Control Mean 0.00 0.02 0.01 0.04 0.02 0.09 0.13 0.18 0.11
SD 0.00 0.06 0.04 0.12 0.05 0.15 0.14 0.23 0.13
Min 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Max 0.00 0.22 0.13 0.55 0.20 0.64 0.45 1.00 0.41
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The general trend shows the error rate increasing in both groups as sentence length and complexity increases.